Physical Factors
Southern California is largely flat, with no really impressive mountain ranges, and obviously well... it's further south. The moist air masses usually pass north of us, just because of how air currents in this part of the world flow. Warm air holds onto its moisture very well. Thus... we're very dry down here.

Northern California has a number of rows of pretty good mountains, the moist air flows directly into the area, and as it hits he mountains, it rises and cools, dropping most of its water as both rain and snow. This is the western part of the "rain shadow effect." So in the north, water is not only dropped by being north and cold, but it's collected and funnelled by the mountains.

The Central Valley / San Joachim Valley is in between the extremes, which is what makes it the "Bread Basket of the United States," and almost the entire world. More than half of the entire US food supply (agriculture, dairy, and livestock) comes out of our Central Valley.

Where do they get it from?
Southern California, home to half of the state's population, depends on the State Water Project, the Colorado River Aqueduct and the Los Angeles Aqueduct supply for about half of its supply. California's vast agricultural industry is also dependent on water projects, both large and small.

The California State Water Project is the largest multipurpose, state-built water project in the United States. The SWP transports water from the Feather River watershed to agriculture, and some of the water goes to industrial and urban users. More than two-thirds of Californians receive some water from the SWP. The system was designed and contracted to deliver 4,200,000 acre feet but in an average year delivers only 2,300,000 acre feet because many of the original planned features were never built.

The CVP’s original purpose was to tame seasonal flooding and to direct water to the south to irrigate 3 million acres of farmland. The CVP is operated by the United States Bureau of Reclamation. As one of the largest water systems in the world it stores over 7,000,000 acre feet of water, or 17 percent of the state’s developed water.

The North Bay Aqueduct of the California State Water Project delivers an annual average of 39,309 acre-feet of water to urban communities and agricultural users in Napa, Solano, Sonoma and Marin counties. That water is diverted from the Sacramento-San Joaquin River Delta Estuary, a water hub that serves as the junction of south-, west, and north-flowing rivers draining the Cascade and Sierra Nevada mountain ranges.